Buggy code:
public void detachModels ( ) { } public final void error ( final Serializable message ) { } public final void fatal ( final String message ) { } public final MarkupContainer findParent ( final Class c ) { } public final MarkupContainer findParentWithAssociatedMarkup ( ) { } public final Application getApplication ( ) { } public final List getBehaviors ( ) { } public final String getClassRelativePath ( ) { } public final IConverter getConverter ( ) { } public IConverter getConverter ( Class type ) { } public final boolean getEscapeModelStrings ( ) { } public final FeedbackMessage getFeedbackMessage ( ) { } public String getId ( ) { } public Locale getLocale ( ) { } public final Localizer getLocalizer ( ) { } public final ValueMap getMarkupAttributes ( ) { } public String getMarkupId ( ) { } final boolean hasMarkupIdMetaData ( ) { } final void setMarkupIdMetaData ( String markupId ) { } public final Serializable getMetaData ( final MetaDataKey key ) { } public IModel getModel ( ) { } public final Object getModelObject ( ) { } public final String getModelObjectAsString ( ) { } public final Page getPage ( ) { } public final IPageFactory getPageFactory ( ) { } public final String getPageRelativePath ( ) { } public final MarkupContainer getParent ( ) { } public final String getPath ( ) { } public final boolean getRenderBodyOnly ( ) { } public final Request getRequest ( ) { } public final RequestCycle getRequestCycle ( ) { } public final Response getResponse ( ) { } public final Session getSession ( ) { } public long getSizeInBytes ( ) { } public final String getString ( final String key ) { } public final String getString ( final String key , final IModel model ) { } public final String getString ( final String key , final IModel model , final String defaultValue ) { } public final String getStyle ( ) { } public String getVariation ( ) { } public final boolean hasErrorMessage ( ) { } public final boolean hasFeedbackMessage ( ) { } public final void info ( final String message ) { } public final boolean isActionAuthorized ( Action action ) { } public final boolean isAncestorOf ( final Component component ) { } public final boolean isEnableAllowed ( ) { } public boolean isEnabled ( ) { } public boolean isVersioned ( ) { } public boolean isVisible ( ) { } public final boolean isVisibleInHierarchy ( ) { } public final void modelChanged ( ) { } public final void modelChanging ( ) { } public final Page newPage ( final Class c ) { } public final Page newPage ( final Class c , final PageParameters parameters ) { } public final void redirectToInterceptPage ( final Page page ) { } public final void remove ( ) { } public final void render ( ) { } public final void render ( final MarkupStream markupStream ) { } public final void renderComponent ( ) { } public final void renderComponent ( final MarkupStream markupStream ) { } public final void rendered ( ) { } private void notifyBehaviorsComponentRendered ( ) { } private void notifyBehaviorsComponentBeforeRender ( ) { } public final void detachBehaviors ( ) { } public void renderHead ( final HtmlHeaderContainer container ) { if ( isVisible ( ) ) { if ( ( this ) instanceof IHeaderContributor ) { ( ( IHeaderContributor ) ( this ) ) . renderHead ( container . getHeaderResponse ( ) ) ; } if ( ( this . behaviors ) != null ) { final Iterator iter = this . behaviors . iterator ( ) ; while ( iter . hasNext ( ) ) { IBehavior behavior = ( ( IBehavior ) ( iter . next ( ) ) ) ; <START_BUG> if ( behavior instanceof IHeaderContributor ) { <END_BUG> ( ( IHeaderContributor ) ( behavior ) ) . renderHead ( container . getHeaderResponse ( ) ) ; } } } } } public void replaceWith ( Component replacement ) { } public final boolean sameRootModel ( final Component component ) { } public final boolean sameRootModel ( final IModel model ) { } public final Component setEnabled ( final boolean enabled ) { } public final Component setEscapeModelStrings ( final boolean escapeMarkup ) { } public final void setMetaData ( final MetaDataKey key , final Serializable object ) { } public Component setModel ( final IModel model ) { } public final Component setModelObject ( final Object object ) { } public final Component setOutputMarkupId ( final boolean output ) { } public final boolean getOutputMarkupId ( ) { } public final void setRedirect ( final boolean redirect ) { } public final Component setRenderBodyOnly ( final boolean renderTag ) { } public final void setResponsePage ( final Class cls ) { } public final void setResponsePage ( final Class cls , PageParameters parameters ) { } public final void setResponsePage ( final Page page ) { } public Component setVersioned ( boolean versioned ) { } public final Component setVisible ( final boolean visible ) { } public final Component setOutputMarkupPlaceholderTag ( final boolean outputTag ) { } public final Object visitParents ( final Class c , final Component . IVisitor visitor ) { } public String toString ( ) { } public String toString ( final boolean detailed ) { } public final CharSequence urlFor ( final Class pageClass , final PageParameters parameters ) { } public final CharSequence urlFor ( final IRequestTarget requestTarget ) { } public final CharSequence urlFor ( final IPageMap pageMap , final Class pageClass , final PageParameters parameters ) { } public final CharSequence urlFor ( final RequestListenerInterface listener ) { } public final CharSequence urlFor ( final IBehavior behaviour , final RequestListenerInterface listener ) { } public final CharSequence urlFor ( final ResourceReference resourceReference ) { }

Fix:
if ( ( behavior instanceof IHeaderContributor ) && ( behavior . isEnabled ( this ) ) ) {